Bring a large pot of lightly salted water to the boil. Add spaghetti and cook for 8 to 10 minutes or until al dente. Drain and set aside.
In a large frying pan over medium-high heat, saute the onions, green peppers and minced beef. Once beef is brown and onions and peppers softened, add the tomatoes, mushrooms and oregano. Simmer uncovered for 10 minutes.
Preheat oven to 180 C / Gas 4. Grease a 20x30cm baking dish. Place half of the cooked spaghetti into the prepared dish. Top with half of the meat mixture. Sprinkle with half of the Cheddar. Repeat. In a medium bowl, mix together cream of mushroom soup and water until smooth; pour over the top and sprinkle with Parmesan cheese.
Bake in preheated oven for 30 to 35 minutes. Let stand 10 minutes before serving.